Hi all.
I was wondering if some one could explain about density and
gravity of gemstones as it seems that through reading the
archives that quite a few people are able to identify stones
based on their gravity and some formula , which was not given in
any detail in the archives…?
Any ideas…?
Greg Efendakis
Fendakis Investments
Zambia Limited
For on specific gravity and gems read the article
at:
Charles
Every stone has a specific gravity, which is a mathematical
number for how many times heavier it is than an equal amount of
water H2O. There liquids that you can put the stones into of
different liquids with varying densities/specific gravities. If
the stones sink they have a greater specific gravity than that
liquid, if they float they have a lesser specific gravity and if
the are suspended in the liquid then they have the same specific
gravity as the liquid. Through a process of elimination you can
figure out that the material is more than X and less than Y.
This most often along with some other tests: refraction, color,
hardness, transparency can give you a very good idea of what you
have as a gem material.
Thanks,
Etienne Perret
Greg, What you’re asking about is Specific Gravity which is the
same thing as density - i.e. the relative massiveness of a
particular material. You know, one cubic centimeter of silver is
heavier than one cubic centimeter of aluminum, but lighter than
one cubic centimeter of lead, and so on. The Specific Gravity of
a material is the ratio of the weight of a given volume of that
material when weighed in air to the weight when weighed in
water. This is a useful number that can be used to help identify
and differentiate among materials - for instance, gemstones
(cubic zirconium is about 60% heavier than diamond.) Tables of
specific gravities are available in many reference books. There
are specific gravity testing liquids available such that stone A
will float in it, but a simulant will sink, or vice versa. Ain’t
Science wonderful??!!
The term you are looking for is “Specific Gravity”. Density,
although related is a different measurement. It goes something
like this: Density is a ratio between a material’s MASS and it’s
Volume. It is related to atomic weights and numbers and is
expressed as grams/cubic centimeter. Specific gravity is also a
ratio, but in this case, between the weight of your gemstone and
the weight of an equal volume of water. This number does not have
units. Specific gravity can be found by several methods. One is
to measure the stone’s weight in AIR, then take it’s weight in
WATER. (Suspend the stone on a thread tied to your scale and
immerse it in water). Then use the formula: WA/ WA-WW or,
weight in AIR divided by the weight in Air minus the weight in
water. Hope this helps. Will Estavillo www.natureshop-gallery.com
For Greg Efendakis, Specific Gravity is defined as: S.G.= Weight
of an object in air/Weight of an equal volume of water. It’s easy
enough to weigh the stone in air (the numerator), so the only
trick is how to find the weight of an equal volume of water.
Archimedes discovered how to do this, reportedly while soaking
in his bath tub. Archimedes found that a body totally submerged
in water will be buoyed up by a force equal to the weight of
water displaced by the object. Therefore, the LOSS IN WEIGHT of
the stone is exactly equal to the weight of the water displaced
by the stone. (That’s the denominator in the equation above). Now
you have all the you need to determine the S.G. We
can write te equation as follows: S.G.= Weight in air/Weight in
air minus Weight in water Example: A certain stone weighs 12.00
grams when weighed in air. When suspended in water, the stone
weighs only 9.00 grams. Our equation would say: S.G. =
12.00/12.00 - 9.00 or 12.00/3.00 = 4.00 This may indicate
Corundum (S.G.= 3.9-4.1) but not Quartz (S.G.= 2.65) Lots of
stones have close to the same S.G., so this property should be
used as a GUIDE, not an absolute identifier, of a stone Some
final notes on weighing technique. The smaller the stone the
more sensitive the BALANCE must be. (You can’t conveniently use a
single pan scale because you need something to suspend the stone
from, while it is submerged in water.) Suspend the stone from the
finest wire or thread you can conveniently manipulate and
submerge as little excess thread as possible (because anything
submerged that is not the stone will will cause a slight error)
Now go for it! …Bob Williams
I was wondering if some one could explain about density and gravity of gemstones .....
G’day Greg Efendakis; Density is the weight of an object
compared with the weight of an equal volume of water. (at 4 C) It
is also called the Specific Gravity (SG). It can also be
described as the weight of an object in air divided by it’s
weight in water. (Archimedes Principle) But when you have tiny
stones it is difficult to suspend them in water to weigh them.
So a gemmologist has at hand a series of little bottles
containing liquids of slightly differing SG’s into which he
places a stone. In one liquid it will just float, and in the next
one it will just sink, so the SG is between the two. As it is
possible to get the sg’s of the liquids very close to each other
and very precisely known, this is quite an accurate method of
determining the density of a stone. However, a gemmologist would
not rely only upon the SG of a stone for identification, but
would have many other tests available.
Hello Greg, The specific gravity is the number which is the ratio
between the weight of the stone and the weight of the equal
volume of water at 4 C. It means a stone with a specific gravity
of 3.52 weighs 3.52 times as much as the same volume of water.
(You need a hydrostatic balance. I have a small electronic scale
and I have bought a hydrostatic weight attachement for it from
GIA Gem Instruments) It is calculated like this: weight in air
Specific gravity = ______________________ X 1 (this is the SG
of water at 4 C) weight in air - weight in water
For example;
You have an unknown stone.You weigh it out in the air.You get 4.61ct. You
then weigh the stone in water 3.41ct
4.61
SG = _________ X 1 = 3.84 Your stone is a garnet.
4.61 - 3.41
I hope I made sense. Kind regards from Turkey, Istanbul Oya
Borahan www.alisails.simplenet.com/oya
listen up people - forget about the 'density-smensity’
follderol; if you’re going to be handling a lot of high-end
arcane stones of unknown composition it’s doubtful you will get
an in-depth education online: go to a gem identification course.
if, however, you want to be able to identify everyday bread &
butter stones, then run barefoot through them until you can tell
from 4 feet away which is what & can tell if it’s genuine from 2
feet away; that part is learned only ‘hands on’ (or feet on).
when it comes to using the 'relative density/specific gravity’
method, few clients will let you remove their stones from
settings so you can weigh it in a glass of liquid. get a copy of
Richard T. Liddicoat, Jr’s HANDBOOK OF GEM IDENTIFICATION - a
GIA publication. no barefoot stone setter/cutter should be
without one. ta - ive
Hello Greg, The specific gravity is the number which is the ratio
between the weight of the stone and the weight of the equal
volume of water at 4�C. It means a stone with a specific gravity
of 3.52 weighs 3.52 times as much as the same volume of water.
(You need a hydrostatic balance. I have a small electronic scale
and I have bought a hydrostatic weight attachement for it from
GIA Gem Instruments) It is calculated like this:
weight in air
Specific gravity = ______________________ X 1 (this is the SG of water
at 4�C)
weight in air - weight in water
For example;
You have an unknown stone. You weigh it out of water. You get
4.61ct. You then weigh the stone in water 3.41ct
4.61
SG = _________ X 1 = 3.84 Your stone is a garnet.
4.61 - 3.41
I hope I made sense. Kind regards from Turkey, Istanbul Oya
Borahan www.alisails.simplenet.com/oya
Ive, You made some very valid points about gemstone
identification, but the original inquiry was by a person without
any experience who was attempting to identify gems in the
rough…African, to be more specific. African gems often tend
to have lost their original form, therefore, it behooves the
identifier to seek other ways to i.d. them. Specific gravity is
a valid tool but, one should never rely on just one physical or
optical characteristic to identify any mineral substance (
except, maybe, diamonds )Your generalizations about sight
identification at four feet are bogus. True, some synthetics
leap out at you…dichroic corundum Alexandrite imitations,
for example.On the other hand, I challenge you to pick out the
synthetic rubies from a parcel of melee size stones without
magnification! The same would be true of many parcels of
amethyst. My own approach is to examine a stone at a couple of
feet and if I cannot be sure about it, go to my venerable old
binocular microscope and check it out at ten power
magnification. If you know your stones (characteristic flaws and
inclusions) the 'scope will make short schrift of phonies. If
the stone is super clean and does not show characteristic
natural aberrations or, if it does not have the distinguishing
characteristics of synthetized stones or simulants, then you
must start using the myriad tests which would be definititive
and appropriate for the stone in question. Ultimatley, paranoia
is the best tool for identifying stones in cut form. The
technology is available to reproduce almost any mineral
substance INCLUDING natural aberrations which were once thought
to be conclusive evidence of origin. You will note that over the
past twenty years supplier catalogs have proliferated in the
offering of synthetics. I hate to admit it, but I think that in
the fairly near future the extraction of natural gemstones will
be obsolete because there will be no effective way of
differentiating them synthetics, therefore, no economic
incentive. I am placing my bets on semi -precious stones which
don’t need to be “sparkly” Nearly every piece of Montana moss
agate is a work of natural art, etc. etc. Ron at Mills Gem, Los
Osos, CA.